Gate Drivers and Voltage Regulators for Gallium Nitride Devices and Integrated Circuits

ABSTRACT

Voltage stabilizing and voltage regulating circuits implemented in GaN HEMT technology provide stable output voltages suitable for use in applications such as GaN power transistor gate drivers and low voltage auxiliary power supplies for GaN integrated circuits. Gate driver and voltage regulator modules include at least one GaN D-mode HEMT (DHEMT) and at least two GaN E-mode HEMTs (EHEMTs) connected together in series, so that the at least one DHEMT operates as a variable resistor and the at least two EHEMTs operate as a Zener diode that limits the output voltage. The gate driver and voltage regulator modules may be implemented as a GaN integrated circuits, and may be monolithically integrated together with other components such as amplifiers and power HEMTs on a single die to provide a GaN HEMT power module IC.

RELATED APPLICATIONS

This application claims the benefit of the filing date of ApplicationNo. 62/690,378, filed on Jun. 27, 2018, Application No. 62/694,663,filed on Jul. 6, 2018, and application Ser. No. 16/449,356, filed onJun. 22, 2019, the contents of which are incorporated herein byreference in their entirety.

FIELD

This invention relates generally to gallium nitride (GaN) based powerelectronic devices and integrated circuits. More specifically, theinvention relates to gate drivers and voltage regulators for GaN powertransistors and devices, and implementations on GaN integrated circuits.

BACKGROUND

Wide bandgap (WBG) gallium nitride (GaN) based power electronic devicesare gaining importance as next generation high-efficiency power devicesowing to superior material properties such as high electric breakdownfield, high electron saturation velocity, and high electron mobility ina readily available heterojunction 2-D electron gas (2DEG) channel. Withthis technology it is possible to achieve much higher power density aswell as efficiency for power electronic devices.

At present, available GaN platforms include GaN on silicon, GaN onsapphire, and GaN on GaN. Regardless the substrate material used, acommon challenge is the mismatch of the gate driving voltage, being muchlower than its silicon counterpart, making it difficult to directlyreplace silicon MOSFETs or IGBTs in existing power electronic systems. Alower gate driving voltage makes GaN devices less immune to drivingvoltage noise and thus reduces system reliability.

One common solution for overcoming this mismatch is to use low voltageMOSFETs as the front end in a cascode configuration. However, such anapproach suffers from mismatch of channel leakage currents between thetwo different transistors, which degrades the reliability andperformance of such cascoded devices compared to high electron mobilitytransistor (HEMT) counterparts.

SUMMARY

One aspect of the invention relates to a gate driver circuit for agallium nitride (GaN) power high electron mobility transistor (HEMT),comprising: an input point that receives an input voltage and an outputpoint that outputs an output voltage to drive the power HEMT; a seriescircuit comprising at least one GaN D-mode HEMT (DHEMT) and at leastfirst and second GaN E-mode HEMTs (EHEMTs); wherein: a drain of theDHEMT is connect to the input point and a source of the DHEMT isconnected to a drain of the first EHEMT and to the output point; a gateof the DHEMT is connected to the source of the DHEMT; a source of thefirst EHEMT is connected to a drain of the second EHEMT; a gate of thefirst EHEMT is connected to the drain of the first EHEMT; a source ofthe second EHEMT is connected to a circuit common; and a gate of thesecond EHEMT is connected to the drain of the second EHEMT.

In one embodiment the gate driver circuit provides voltage down-shiftingand over-voltage protection to drive the GaN power HEMT.

In one embodiment the at least one DHEMT operates as a variable resistorand the at least first and second EHEMTs operate as a zener diode thatlimits the output voltage to about 6 V.

In one embodiment the DHEMT and the at least first and second EHEMTs arelow voltage devices.

In one embodiment the DHEMT and the at least first and second EHEMTshave different channel lengths and/or different channel widths.

In one embodiment the DHEMT and the at least first and second EHEMTshave the same channel lengths.

In one embodiment the DHEMT has a channel width that is 17 to 25 percentof the channel width of a smallest of the at least first and secondEHEMTs.

In one embodiment the channel widths of the at least first and secondEHEMTs are the same, and a ratio of channel widths of the DHEMT:EHEMTsis about 1:4.7.

Another aspect of the invention relates to a GaN integrated circuit (IC)power module, comprising: a gate driver circuit as described herein; anda power HEMT; wherein the gate driver module and the power HEMT aremonolithically integrated in a single die.

In one embodiment the GaN power module power HEMT comprises at leastfirst and second high voltage HEMTs connected together in parallel; thefirst high voltage HEMT is smaller than the second high voltage HEMT sothat an area of the die is not occupied by the power HEMT; and the gatedriver module is disposed in the area of the die that is not occupied bythe power HEMT.

Another aspect of the invention relates to a method for implementing agate driver circuit for a GaN power HEMT, comprising: providing an inputpoint that receives an input voltage and an output point that outputs anoutput voltage to drive the power HEMT; connecting at least one GaNDHEMT and at least first and second GaN EHEMTs together in series;wherein: a drain of the DHEMT is connect to the input point and a sourceof the DHEMT is connected to a drain of the first EHEMT and to theoutput point; a gate of the DHEMT is connected to the source of theDHEMT; a source of the first EHEMT is connected to a drain of the secondEHEMT; a gate of the first EHEMT is connected to the drain of the firstEHEMT; a source of the second EHEMT is connected to a circuit common;and a gate of the second EHEMT is connected to the drain of the secondEHEMT; wherein a gate driver circuit for a GaN power HEMT is provided.

In one embodiment the method comprises monolithically integrating thegate driver together with the power HEMT in a single GaN die. In oneembodiment the power HEMT is implemented using at least first and secondhigh voltage HEMTs connected together in parallel; wherein the firsthigh voltage HEMT is smaller than the second high voltage HEMT so thatan area of the die is not occupied by the power HEMT; and the gatedriver module is disposed in the area of the die that is not occupied bythe power HEMT.

Another aspect of the invention relates to a voltage regulator circuitfor a GaN device, comprising: an input point that receives an inputvoltage and an output point that outputs an output voltage for the GaNdevice; a series circuit comprising at least one GaN D-mode HEMT (DHEMT)with gate-to-source connection and at least first to fourth GaN E-modeHEMTs (EHEMTs) each with drain-to-gate connection; an output EHEMThaving a drain connected to the input point and a source connected tothe output point; wherein: a drain of the DHEMT is connected to theinput point and a source of the DHEMT is connected to a drain of thefirst EHEMT and to a gate of the output EHEMT; a source of the firstEHEMT is connected to a drain of the second EHEMT, a source of thesecond EHEMT is connected to a drain of the third EHEMT, a source of thethird EHEMT is connected to a drain of the fourth EHEMT, and a source ofthe fourth EHEMT is connected to a circuit common.

In one embodiment the voltage regulator circuit provides a referencevoltage output.

In one embodiment the GaN device comprises an amplifier.

In one embodiment the at least one DHEMT operates as a variable resistorand the at least first to fourth EHEMTs operate as a Zener diode thatlimits the output voltage to about 6 V.

In one embodiment the DHEMT and the at least first to fourth EHEMTs arelow voltage devices.

In one embodiment the DHEMT and the at least first to fourth EHEMTs havedifferent channel lengths and/or different channel widths.

In one embodiment the DHEMT has a channel width that is about 2 to about20 percent of the channel width of a smallest of the at least first tofourth EHEMTs.

In one embodiment, channel widths of the at least first to fourth EHEMTsare the same.

Another aspect of the invention relates to a GaN integrated circuitpower module, comprising: a voltage regulator circuit as describedherein; an amplifier; and a power HEMT; wherein the voltage regulatorcircuit, the amplifier, and the power HEMT are monolithically integratedin a single die.

In one embodiment, the GaN IC power module power HEMT comprises at leastfirst and second high voltage HEMTs connected together in parallel;wherein the first high voltage HEMT is smaller than the second highvoltage HEMT so that an area of the die is not occupied by the powerHEMT; and the voltage regulator circuit and the amplifier are disposedin the area of the die that is not occupied by the power HEMT.

Another aspect of the invention relates to a method for implementing avoltage regulator circuit for a GaN device, comprising: providing aninput point that receives an input voltage and an output point thatoutputs an output voltage to drive the GaN device; connecting at leastone GaN D-mode HEMT (DHEMT) with gate-to-source connection in serieswith at least first to fourth GaN E-mode HEMTs (EHEMTs) each withdrain-to-gate connection; connecting a drain of an output EHEMT to theinput point and a source of the output HEMT to the output point;wherein: a drain of the DHEMT is connected to the input point and asource of the DHEMT is connected to a drain of the first EHEMT and to agate of the output EHEMT; a source of the first EHEMT is connected to adrain of the second EHEMT, a source of the second EHEMT is connected toa drain of the third EHEMT, a source of the third EHEMT is connected toa drain of the fourth EHEMT, and a source of the fourth EHEMT isconnected to a circuit common; wherein a voltage regulator circuit for aGaN device is provided.

In one embodiment the method comprises monolithically integrating thevoltage regulator circuit together with an amplifier and a power HEMT;wherein the voltage regulator circuit, the amplifier, and the power HEMTare monolithically integrated in a single die.

In one embodiment the method comprises: implementing the power HEMT withat least first and second high voltage HEMTs connected together inparallel; wherein the first high voltage HEMT is smaller than the secondhigh voltage HEMT so that an area of the die is not occupied by thepower HEMT; and the voltage regulator circuit and the amplifier aredisposed in the area of the die that is not occupied by the power HEMT.

Another aspect of the invention relates to a GaN based gate-inputintegrated circuit consisting of two to four EHEMTs and one DHEMTconnected in series (source to drain) connected such that the gate ofthe EHEMT is connected to the drain electrode; the gate of the DHEMT isconnected to the source electrode; the input of the IC is the drain ofthe DHEMT; and the output of the IC is the source of the DHEMT. Theoutput may be used as a gate driver for at least one GaN powertransistor.

In one embodiment the DHEMT is monolithically integrated with multipleEHEMTs of different channel lengths and widths, and all components havelower voltage rating than the power transistor(s).

In one embodiment the DHEMT is integrated with the EHEMTs using the sametechnology with the same channel length while the DHEMT channel width isabout 17 to about 25 percent of that of the smallest EHEMT.

In one embodiment the EHEMTs are implemented in a two-metal designhaving the following arrangements: a gate Metal2 is placed parallel tothe EHEMT gate fingers; the gate Metal2 is positioned on the sides nextto the drain electrode (gate side-bar); and a short Metal2 plateconnects the side-bar and the drain electrode.

In one embodiment the main power transistor and the gate-input IC arearranged such that the main power transistor comprises two powertransistors connected in parallel and fingers of the two powertransistors are parallel; the fingers of one of the power transistorsare shorter than the those of other; one edge of the shorter powertransistor is aligned with an edge of the longer power transistor whileanother edge of the shorter power transistor defines a rectangular waferspace on the die; the source and drain arrangement of the two powertransistors is such that the rectangular wafer space of the die issurrounded by the source electrode of the two power transistors; thegate input driver multiple transistors (DHEMT and EHEMTs) connected inseries are disposed within the rectangular wafer space.

In one embodiment the DHEMT of the gate input IC is asymmetric with thedrain being larger to accommodate a wire bonding pad or a land gridarray (LGA) or ball grid array (BGA) metal bump.

Another aspect of the invention relates to a GaN based auxiliary voltageregulator integrated circuit including four to six EHEMTs connected inseries with one DHEMT (source to drain) and one larger size EHEMT inparallel with the DHEMT, and with a connection such that the gate of asmaller EHEMT is connected to the drain electrode; the gate of the DHEMTis connected to source electrode; a larger EHEMT has its gate connectedto the source of the DHEMT and its drain connected to drain of theDHEMT; wherein the input of the IC is the drain of the DHEMT; and theoutput of the IC is the source of the larger EHEMT.

In one embodiment the DHEMT is monolithically integrated with multipleEHEMTs of different channel lengths and widths, and the larger EHEMT isthe largest in width, and all IC components have lower voltage ratingthan a main power transistor(s).

In one embodiment the EHEMTs and the DHEMT are integrated using the sametechnology with the same channel length while the DHEMT channel width isabout 2 to about 20 percent of that of the smallest EHEMT.

In one embodiment the smaller EHEMTs have an arrangement such that theirgate fingers are parallel with all finger tips aligned to form a singlecolumn (or row); each EHEMT has a source and drain flipped (i.e.,alternating sides) relative to adjacent EHEMTs; and the source and drainelectrodes of each EHEMT are connected using Metal2 layer to theopposite electrode of its adjacent EHEMT.

In one embodiment, an output EHEMT is larger and is arranged such thatits gate fingers are perpendicular to the smaller EHEMTs and its smallerdimension is the same as an array width of the smaller EHEMTs, such thatthe complete regulator layout forms a rectangular block.

In one embodiment the main power transistor and the regulatorrectangular block are arranged such that the main power transistorcomprises two power transistors connected in parallel; wherein fingersof the two power transistors are parallel; the fingers of one powertransistor are shorter than those of the other; one edge of the shorterpower transistor is aligned with an edge of the longer power transistorwhile the another edge of the shorter power transistor defines arectangular space on the wafer die; wherein the source and drainarrangement of the two power transistors are such that the rectangularspace is surrounded by the source electrode of the two powertransistors; wherein the four to six EHEMTs and the DHEMT of the voltagerectangle block are placed on the outer edge of the rectangular space.

Another aspect of the invention relates to a gate driver circuit for agallium nitride (GaN) power transistor, comprising: an input point thatreceives an input voltage and an output point that outputs an outputvoltage to drive the GaN power transistor; a series circuit comprisingat least one GaN D-mode HEMT (DHEMT) with gate-to-source connection andat least first to fourth GaN E-mode HEMTs (EHEMTs) each withdrain-to-gate connection; an output EHEMT having a drain connected tothe input point and a source connected to the output point; wherein adrain of the DHEMT is connected to the input point and a source of theDHEMT is connected to a drain of the first EHEMT and to a gate of theoutput EHEMT; wherein a source of the first EHEMT is connected to adrain of the second EHEMT, a source of the second EHEMT is connected toa drain of the third EHEMT, a source of the third EHEMT is connected toa drain of the fourth EHEMT, and a source of the fourth EHEMT isconnected to a circuit common.

In one embodiment, the output EHEMT comprises a source followeramplifier.

In one embodiment, the gate driver circuit further comprises anamplifier; wherein the output point is connected to an input of theamplifier; wherein an output of the amplifier drives the GaN powertransistor.

In one embodiment, the amplifier input comprises a bootstrapconfiguration. The bootstrap configuration may include a diode connectedin series between the output point and the amplifier input, and acapacitor connected between the amplifier input and the circuit common.

In one embodiment, the amplifier comprises a direct coupled FET logic(DCFL) amplifier.

In various embodiments, the gate driver circuit may comprise a GaN powertransistor; wherein the GaN power transistor is self-driven.

Embodiments of the gate driver circuit may be implemented in GaNintegrated circuit technology. Embodiments may be implemented in GaNintegrated circuit technology according to methods and techniquesdescribed herein.

Another aspect of the invention relates to a power converter comprisingone or more GaN power switches; wherein at least one GaN power switchcomprises a gate driver circuit as described herein, wherein the GaNpower switch is self-driven. The GaN power switch may be a synchronousrectifier of a power converter.

Another aspect of the invention relates to methods for gate drivers fora gallium nitride (GaN) power transistor as described herein. Themethods may include implementations in GaN integrated circuittechnology, and implementations in power converters wherein one or moreGaN power switch of a power converter is self-driven.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, and to show more clearlyhow it may be carried into effect, embodiments will be described, by wayof example, with reference to the accompanying drawings, wherein:

FIG. 1 is a schematic diagram showing a gate driving circuit for a GaNpower transistor, according to one embodiment.

FIG. 2 is a schematic diagram showing an equivalent circuit of a voltagestabilizing circuit of the embodiment of FIG. 1.

FIG. 3A is a schematic diagram illustrating how an increase in draincurrent can cause a negative voltage feedback which is used to stabilizethe gate voltage for a GaN power transistor.

FIG. 3B is a plot showing electron velocity versus field of a D-modehigh electron mobility transistor (DHEMT) channel.

FIG. 3C is a plot showing the drain current-voltage characteristic ofthe DHEMT M3 at zero gate voltage.

FIG. 4 is a diagram showing a layout for an E-mode HEMT using two metallayers (Metal1 and Metal2), according to the prior art.

FIG. 5 is a diagram showing a layout for a Dmode HEMT using two metallayers (Metal1 and Metal2) with the source and gate shorted, accordingto one embodiment.

FIG. 6 is a diagram showing a layout for an Emode HEMT with the gate anddrain shorted, according to one embodiment.

FIG. 7 is a diagram showing a layout for a gate input IC module near thesource side of a main GaN power transistor, according to one embodiment.

FIG. 8 is a diagram showing a detailed view of the layout of the gateinput IC module near the source side of the main GaN power transistor ofthe embodiment of FIG. 7, wherein two high voltage main powertransistors of rectangular shape are connected in parallel toaccommodate the gate input IC.

FIG. 9 is a diagram showing a detailed view of the layout of the DHEMTof the gate input IC module near the source side of the main GaN powertransistor, according to one embodiment.

FIG. 10 is a diagram showing a detailed view of the layout of the gateinput IC module near the source side of the main GaN power transistoraccording to one embodiment wherein gate metal bars of the maintransistor are oriented in the horizontal direction.

FIG. 11 is a plot showing a voltage transfer characteristic of gatedriver modules according to two embodiments, obtained from a simulation.

FIG. 12 is a plot showing current-voltage characteristics of gate drivermodules according to two embodiments, obtained from a simulation.

FIG. 13 is a plot showing pulsed voltage responses of two gate drivermodules with different DHEMT gate widths (size1: DHEMT width=75 μm;size2: DHEMT width=150 μm), obtained from a simulation.

FIG. 14 is a plot showing a pulsed output response of a main EHEMTrelative to an input driving pulse for two gate driver module designs,obtained from a simulation.

FIG. 15 is a plot showing pulsed current response of two gate drivermodules with different gate widths, obtained from a simulation.

FIG. 16 is a schematic diagram of a gate driver circuit, according toanother embodiment.

FIG. 17 is a schematic diagram showing an equivalent circuit of avoltage stabilizing circuit of the embodiment of FIG. 16.

FIG. 18 is a diagram showing a layout of a gate-drain shorted EHEMT,according to one embodiment.

FIG. 19 is a diagram showing a layout of the regulator module of FIG.16, according to an embodiment arranged in a rectangular block.

FIG. 20 is a detailed view of the layout of the four EHEMTs of thelayout embodiment of FIG. 19.

FIG. 21 is a detailed view of the layout of the DHEMT of the layoutembodiment of FIG. 19.

FIGS. 22A and 22B are plots of voltage transfer characteristics andcurrent voltage characteristics of the voltage reference branch of theregulator module, respectively, obtained from a simulation.

FIG. 23 is a schematic diagram showing how the regulator module may beused as an auxiliary power supply for a gate driver, according to oneembodiment.

FIG. 24 is a diagram showing a layout of the auxiliary power supply ofFIG. 23, according to an embodiment for driving a single power EHEMT.

FIG. 25 is a diagram showing a layout of the auxiliary power supply ofFIG. 23, according to an embodiment for driving two power EHEMTs inparallel.

FIG. 26 is a diagram showing a layout of the auxiliary power supply ofFIG. 23, according to another embodiment for driving two power EHEMTs inparallel.

FIGS. 27A and 27B are plots showing input voltage pulse and auxiliaryvoltage output, respectively, obtained from a simulation of theembodiment of FIG. 23 and layout according to FIG. 24, for a main powerEHEMT with nominal maximum current rating of 8 A.

FIGS. 28A and 28B are plots showing gate driving voltage and gatecurrent, respectively, obtained from a simulation of the embodiment ofFIG. 23 and layout according to FIG. 24, for a main power EHEMT withnominal maximum current rating of 8 A.

FIGS. 29A and 29B are schematic diagrams of gate driver circuits for aGaN power transistor, according to embodiments described herein.

FIG. 30 is a schematic diagram of a gate driver circuit for a GaN powertransistor, based on the embodiment of FIG. 29B.

FIGS. 31A and 31B are plots showing performance of a gate driver basedon the embodiment of FIG. 30, obtained from a simulation.

FIG. 32 is a schematic diagram showing an application of a gate driveras described herein as self-driven switches for synchronousrectification in an active clamped converter.

DETAILED DESCRIPTION OF EMBODIMENTS

A drawback of prior GaN power transistor gate drivers based on silicondevices is mismatch of gate driving voltages and channel leakagecurrents between the two different transistor technologies, whichdegrades reliability and performance. For example, the gate drivingvoltage for silicon-based power electronic devices typically ranges from10 to 15 volts. In contrast, the gate driving voltage for GaN HEMTs ismuch lower, ranging from 3 to 6 volts. One aspect of this inventionrelates to voltage stabilizing and/or regulating circuits implemented inGaN HEMT technology that provide stable output voltages suitable for usein applications such as GaN power transistor gate drivers and lowvoltage auxiliary power supplies for GaN devices, circuits, etc.,including GaN integrated circuits, that avoid such mismatches.Embodiments may thus advantageously exploit the 2DEG property of GaNHEMTs. Embodiments described herein may be implemented using GaNdiscrete components, or they may be partially or completely implementedin GaN integrated circuit technology, with low wafer area cost. As anexample, embodiments may be implemented in a 650 V GaN-on-siliconprocess. Emode may be achieved using a p-GaN layer, however, theinvention is not limited thereto as embodiments in Emode may also beachieved using other techniques. Gate driver embodiments may be designedaccording to guidelines of 100 V rules, although any technology rangingfrom 30-100 V is suitable. A two-metal layer process may be used asdescribed herein, although the invention is not limited thereto.

A gate driver module according to one embodiment is shown in theschematic diagram of FIG. 1, where M4 is the main power transistor, andM1, M2, and M3 are the gate driver module (also referred to as “gateinput module”) for M4. M4 is a high voltage (HV) EHEMT (such as, e.g.,650 V to 1200 V), M1 and M2 are two similar or identical low voltage(LV) EHEMTs (such as, e.g., 40 V to 100 V), and M3 is a LV DHEMT. M3 mayuse the same channel/gate feature as M1 and M2. In the embodiment ofFIG. 1 the input driving voltage of the gate driver module Vgi is 10 Vto 30 V and the gate voltage Vg of M4 is 6 V; however, other voltagesare possible.

In the embodiment of FIG. 1, the topology of M1, M2, and M3 providesvoltage down-shifting and over-voltage protection to achieve the propergate driving voltage for M4. The equivalent circuit of M1, M2, and M3 inthe embodiment of FIG. 1 is illustrated in FIG. 2, where the DHEMT M3can be regarded as a variable resistor R while the two EHEMTs M1 and M2can be regarded as a voltage stabilizing Zener diode D, which limits theoutput voltage to the gate of M4 to, e.g., about 6 V.

An advantage of the embodiment of FIG. 1 may be explained by consideringthe electron velocity (proportional to electron current in the DHEMTchannel), as plotted in FIG. 3B. The electron saturation behaviorexhibits negative differential resistance, in theory (Turin, V. O., Amodified transferred-electron high-field mobility model for GaN devicesimulation. Solid-State Electronics 49:1678-1682, 2005). Due to variouschannel scattering mechanisms and for low frequency to DC measurements,the negative resistance is not obtained. Therefore, the usual practiceis to model the electron velocity characteristics as a flat line (asindicated in the plot in FIG. 3B). The flat line means the resistance ofthe DHEMT is variable. At higher bias the resistance is higher, thusreducing the voltage stress for the gate of the main EHEMT M4.

A benefit of using a DHEMT for M3 is also indicated in electron velocityversus field plot (the plot of FIG. 3B) which shows low resistance atlow field or low voltage. This is important since turning off the maintransistor results in a bottleneck in many prior topologies. The highelectron mobility at low electric field (high initial slope in FIG. 3B)makes the DHEMT a good discharging resistor for the main EHEMT M4. Thismay enable a fast and clean turn off for the main power transistor M4.FIG. 3C shows the drain current-voltage characteristic of the DHEMT M3at zero gate voltage.

The EHEMTs M1 and M2 are configured in a unique connection arrangementwith gate-drain shortage. Such a configuration effectively sets up anegative voltage feedback loop as follows. An increase dVg at thedrain/gate (shorted) is amplified into an increase in drain current+Id(see FIG. 3A). The increase in drain current enhances the voltage dropon the load and thus reduces dVg (which is shown as “−dVg1” in FIG. 3A)and this completes the negative feedback.

It is clear from the analysis related to FIG. 3A that +Id is additivewhen more LV EHEMTs are used in series. An ideal embodiment would be touse as many LV EHEMTs as possible for the purpose of voltagestabilization. However, the limit on the number of LV EHEMTs is set bythe maximum Vg of the main power EHEMT. which is typically 6 V for GaNtechnology based on p-GaN fabrication. Assuming a common threshold gatevoltage (Vt) of 1.5-2 V, and given that about 1 V above thresholdvoltage is necessary to bias the LV EHEMT in a proper signalamplification mode, only two LV EHEMTs can be used. However, should theVt be lowered to 1 V or less, three to four LV EHEMTs may be used.

Implementation of the embodiment of FIG. 1 using GaN integrated circuittechnology will now be described. A common two metal-layer technologyEHEMT layout is shown in FIG. 4. The legend shows various layers of thefabrication process, wherein Metal1 and Metal2 are the two top metallayers above the device, used for making electrodes, and Via is anopening through the insulating layers between Metal1 and Metal2 layerswhich is filled with metal for the purpose connecting Metal1 and Metal2.The legend is the same for FIGS. 5-10, 18-21, and 24-26, although inFIGS. 8-10, 19, and 24-26 pad-opening is an opening of the topprotective coating through which electrode contacts are made. In thesefigures, G, D, and S refer to gate, drain, and source, respectively. Thegate metal can be accessed through two metal layers (Metal1 and Metal2,connected through a VIA). The source and drain can only be accessedusing Metal2.

A two metal-layer technology DHEMT layout is shown in FIG. 5, where thegate is shorted to the source for the purpose of providing a stable biasfor the conduction channel. Device fabrication is carried out so as toachieve natural Dmode for the HEMT.

Referring to FIG. 1, the voltage stabilizing EHEMTs M1 and M2 wereimplemented most conveniently using a Metal2 connection from the drainto the gate side-bar, as indicated in FIG. 6. It is noted that the widthratio between LV DHEMT and LV EHEMT must be set carefully to achieve 6 Vstabilized voltage, as described below.

For a power IC layout, placement of the gate driver module relative tothe main power transistor is critical. Several considerations must betaken into account. Firstly, the module must be sufficiently close tothe gate electrode of the main power transistor to reduce parasiticinductance. Secondly, the module must occupy as small an area aspossible to reduce the wafer real estate cost. Thirdly, the module mustbe shielded from the high voltage drain pad to avoid electromagneticinterference.

In FIG. 7, which shows an embodiment of the power module IC layout, thegate driver module placement is shown in the upper left corner relativeto the rest of the power module IC, which is the main GaN transistor M4where the gate fingers are vertically oriented. To provide the waferarea for the gate drive module on the rectangular die, the maintransistor is separated into two rectangular areas of paralleled EHEMTs,thus leaving out the small rectangular area of the required size for thegate driver module, as indicated in FIG. 7. Dashed line A-B separatesthe two rectangle-shaped main transistors connected in parallel as M4.It is noted that the gate driver module IC is close to both of thesource sides of the two main transistors in parallel.

FIG. 8 is a more detailed view of the layout of the gate driver moduleof the embodiment of FIG. 7. The output of the gate driver module isconnected to the top gate rail which is on Metal1 layer (G-Metal1). Theconnection is through a VIA and the drain metal pad of EHEMT M2 which isthe output point of the gate driver module. The Gi pad is the input tothe module and it is on Metal2 layer, shown as Gi-Metal2, and it isnecessary to make this pad large for ease of device packaging. The Gipad is connected to the drain of the DHEMT M3 but isolated electricallyfrom the rest of the devices.

For wire bonding requirements, the Gi contact pad must be sufficientlylarge. Therefore, a unique design is used for the DHEMT M3, as shown inFIG. 9 which is a more detailed view of the layout of FIG. 8. To makethe metal pad of Gi as large as possible, two techniques are used. Thefirst is the upper part (or the left part when the layout of the seriescomponents of the gate driver module is oriented in the horizontaldirection) of the Metal2 gate side-bar is removed to make way for thedrain pad. The second is that the source and drain are implemented in anasymmetric arrangement such that the drain electrode is much larger thanthe source electrode. This way the drain pad can be of a size largeenough for wire bonding or metal bumping.

For land grid array (LGA) or ball grid array (BGA) in a 650 Vapplication, for example, it is necessary to separate the drain andsource pads at a distance larger than about 2 mm. Therefore, in oneembodiment the layout of the main transistor M4 is such that the gatefingers are parallel to the longer direction of the device while thesource and drain are on the two ends of fingers. In another embodimentthe gate fingers may be oriented in the horizontal and longer direction,as shown in FIG. 10.

To reserve space on the die for the gate input module, in one embodimenttwo parallel rectangular transistors are used for the main powertransistor M4, separated by the horizontal line A-B as shown in thepower module layout embodiments of FIGS. 7 and 10. In such anembodiment, the gate input module is located above the larger maintransistor.

To implement embodiments as described herein, a 650V process may be usedfor the main transistor while for the low voltage EHEMTs and DHEMT a100V process may be used, although other processes may also be used. Itis important to design the ratio of the channel width of the DHEMTrelative to those of the EHEMTs to ensure that a correct Vg will beachieved. For example, in embodiments where the two EHEMTs are identicaland the DHEMT is of the same LV technology as the EHEMTs, the channelwidth ratio EHEMT:DHEMT in microns should be about 350μ:75μ, or about4.7:1. Scaling by a constant factor of about the same order as unity(i.e., changing the ratio such as 2:1, 3:1, 8:1, 9:1, etc., where thefirst number is multiplied by a factor of 0.1 to 9 while the 2^(nd)number is always 1) will not alter the results but will affectresistance of the input gate module and thus affect thedischarging/charging current magnitude.

A simulation was performed using APSYS™ software (Crosslight SoftwareInc., Vancouver, Canada; www.crosslight.com) to confirm operation of anembodiment based on the circuit of FIG. 1. In the simulation, two sizeswere considered. Size1 refers to widths of EHEMT:DHEMT 350μ:75μ andSize2 was 7001μ:150μ. The results of Vg vs Vgi are shown in FIG. 11 andboth sizes (Size1 and Size2) gave identical results for voltagedown-shifting.

The biasing current Igi of the gate driver module is sensitive to thedevice channel width. For the channel widths given above, the biasingcurrent Igi is shown in FIG. 12. There is a trade off betweendischarging speed and DC power loss for the IC: a smaller channel widthreduces the DC power loss but slows down the discharging of the maintransistor M4. It is noted that the use of a gate driver module asdescribed herein makes the power module (i.e., the gate driver moduleplus the power transistor M4 (e.g., two main HEMTs in parallel)) behavelike a bipolar junction transistor (BJT) with a current amplification of50-100.

FIG. 13 shows the input/output pulse response of the gate driver modulefor the two channel sizes. The low resistance of the DHEMT results in aturn on/off with a clean edge and without a prolonged tail. As expected,the larger size gate driver module makes a faster switch. The clean turnon/off is also indicated in FIG. 14 which shows the power transistoroutput Vd pulse response relative to the gate driver module inputvoltage for the two channel sizes.

FIG. 15 shows the DC biasing current (thus power loss) of the gatedriver module for the two channel sizes. It is clear that there is atrade off between DC power loss and the gate delay.

Another embodiment is shown in the schematic diagram of FIG. 16, whereM60 is a low voltage (LV) EHEMT (such as, e.g., 40 V to 100 V) with alarger size channel and it supplies the current needed for a gate driverfor a power HEMT, or for driving a device, circuit, etc. M10 to M40 aresmaller LV EHEMTs with identical or similar channel sizes that have aconnection shorting the gate with the drain. M50 is a DHEMT with aconnection shorting the gate with its source. As will be discussed indetail below, M10 to M50 function to provide a reference voltage. Thecircuit may be referred to as a voltage regulator, or a voltageregulator module.

The equivalent circuit of the embodiment of FIG. 16 is shown in FIG. 17,where the DHEMT M50 can be regarded as a variable resistor R while thefour EHEMTs M10 to M40 can be regarded as a voltage stabilizing Zenerdiode which limits the reference voltage at about 7.5V.

The embodiment of FIG. 16 also achieves the advantages discussed abovefor the embodiment of FIG. 1, with respect to electron velocitycharacteristics (see FIG. 3B) and the variable resistance of the DHEMT,i.e., M50 in the embodiment of FIG. 16.

The benefit of using a DHEMT for M50 is also due to the low resistanceat low field or low voltage. The high mobility (high initial slope inupper insert of FIG. 3B) takes the DHEMT quickly into the saturationregime where it acts as a variable resistor. For the same current flow,the voltage drop on the DHEMT can be varied so that any excess voltagefrom Vcci can be taken by the DHEMT M50, ensuring the reference voltageremains at 7.5 V so that current supplying the EHEMT source can beclamped at the required Vcc=6 V.

As in the embodiment of FIG. 1, the EHEMTs M10 to M40 of FIG. 16 areconfigured in a unique contact connection arrangement with gate-drainshortage, which effectively sets up a negative voltage feedback loop asdescribed above.

As in the embodiment of FIG. 1, +Id is additive when more LV EHEMTs areused in series. Whereas it is desirable to use as many LV EHEMTs aspossible for voltage stabilization purposes, the limit on the number ofLV EHEMTs is set by the reference voltage which in this embodiment isabout 7.5 V. Assuming a common threshold gate voltage (Vt) of about 1.5V, and given that about 0.3-0.5 V above threshold voltage (Vt) isnecessary to bias the LV EHEMTs in a proper signal amplification mode,only four LV EHEMTs can be used. However, should the Vt be lowered to 1V or less, five to seven LV EHEMTs may be used.

The current supplying larger EHEMT M60 is connected in a way negativefeedback can be used to stabilize the auxiliary power supply voltageVcc. The negative feedback of M60 works as follows: consider a powersupply load connected between ground and the source of M60. An increasein supply current in M60 would increase the voltage of the source of M60and Vgs would be reduced. A reduced Vgs for M60 would increase the DCresistance of M60 and thus reverse the voltage increase of the source ofM60. This completes the negative feedback loop. It is clear that effectof negative feedback is proportional to the transconductance of M60,which is proportional to the channel width/area of M60. A more stableVcc requires a larger transistor size at higher cost of wafer area.

Implementation of the embodiment of FIG. 16 using GaN integrated circuittechnology will now be described. The two metal-layer technology EHEMTlayout of FIG. 4, and the DHEMT layout of FIG. 5 where the gate isshorted to the source, as described above, may be used in thisembodiment. The voltage stabilizing EHEMTs M10 to M40 may be implementedusing a Metal2 connection from drain to the top gate Metal1, as shown inFIG. 18. It is noted that the width ratio between LV DHEMT and LV EHEMTmust be set carefully to achieve 7.5 V stabilized voltage, as describedbelow.

The layout of the regulator must be carefully planned. Firstly, itshould be sufficiently small to make economic sense. Secondly, it shouldbe placed on the edge of the whole module and not be in the way betweenthe driver and the main power transistor. Thirdly, the power outlet Vccshould be on the top side to conveniently supply the whole drivercircuit, which may include an amplifier as shown in the embodiment ofFIG. 23.

FIG. 19 shows an embodiment of the regulator layout in a rectangularblock. The EHEMTs M10 to M40 are placed at the lower end in the figurewhile the larger M60 is at the upper end. The power outlet is the sourceof M60 and this is located at the top of the die. It is noted that tomake a constant x-size of the rectangular block, the four EHEMTs M10 toM40 have fingers oriented horizontal or perpendicular to those of thelarger M60, as indicated in FIG. 20. With the source and drainelectrodes arranged alternatively from M10 to M40, it is convenient toconnect the drain of one HEMT to the source of the next using a shortMetal2 rectangle.

Compared to the other transistors, the DHEMT M50 is relatively small, ascan be seen in the layout embodiment of FIG. 19, and the more detailedview of FIG. 21. In one embodiment, the ratio of channel widths is suchthat the width of the DHEMT is about 2 to 20 percent, or about 2 to 10percent, or about 3 to 5 percent of that of the EHEMTs (i.e., M10 toM40). If the channel widths of the EHEMTs are not equal but similar, theratio should be based on the smallest EHEMT. To ensure that the gate ofthe DHEMT can be used as a common connector for the entire regulatorwithout occupying a lot of area on the die, the fingers of the DHEMT areoriented perpendicular to those of the larger transistor M60, as shownin the embodiment of FIG. 21.

A simulation was conducted on the embodiment of FIG. 16 with the IClayout of FIG. 19, using APSYS™ software. FIG. 22A shows the simulatedvoltage transfer characteristics of the regulator IC and FIG. 22B showsthe current-voltage characteristics of the voltage reference branch ofthe regulator. It is clear from FIG. 22A that the designed 7.5 V isachieved for a wide range above 8 V, validating the design. As areference voltage provider, it is desirable that the DC current throughthe M10-M40 branch is small so that DC loss is minimal. This puts anupper limit to the sizes (channel widths) of M10 to M50. For example,for a 100 V LV embodiment, the EHEMT channel width is about 400 μm foreach EHEMT.

FIG. 23 is a schematic diagram showing how the regulator may be usedwith an amplifier A, such as a direct-coupled FET logic (DCFL)amplifier, a buffer amplifier, etc., as an auxiliary power supply forgate driver for a main power transistor M70. In various embodiments theregulator, amplifier, and power transistor M70 may be implementeddiscretely, or they may be implemented monolithically integrated in GaNIt is noted that regulating Vcci down to the required 6 V unavoidablycauses power loss which is proportional to the input voltage Vcci andcurrent supplied through transistor M60. Thus, embodiments may use alower system voltage, that is, a voltage at the lower end of 10-30V, andthe reference voltage branch M10 to M40 may be designed to draw a low DCcurrent.

FIG. 24 shows an embodiment of a die layout for the circuit of FIG. 23,including the regulator described above and shown in FIG. 19 in a powerIC. The fingers of the main power HEMT M70 are parallel to the regulatorlonger direction, and the finger length of the main power HEMT isdesigned to be about the same as the longer side of the regulator block.The amplifier-driver block is designed to be about the same height asthe regulator (as shown in FIG. 24) and it is sandwiched between themain power HEMT M70 and the regulator block.

FIGS. 25 and 26 show die layouts for two embodiments including theregulator as in FIG. 19, but with a main power HEMT with larger currentrating. With such a large power HEMT it is not area-efficient to use thesame arrangement as FIG. 24 since the regulator dimensions are muchsmaller than the main power HEMT. In the embodiment of FIG. 25 theregulator block is rotated 90 degrees relative to the main power HEMTwhere the gate fingers are vertically oriented. To provide area on thedie for the regulator and the amplifier/driver block, the main powertransistor is separated into two rectangular areas of paralleled EHEMT,leaving a rectangular area of the required size. Dashed line A-Bseparates the two rectangle-shaped main transistors connected inparallel. It is noted that the LV blocks (i.e., the regulator andamplifier) are located close to the source sides of the two maintransistors in parallel.

As noted above, for LGA or BGA packaging on a 650 V application, it isnecessary to separate the drain and source pads by a distance largerthan 2 mm. Therefore, in one embodiment the layout of the maintransistor is such that the gate fingers are parallel to the longerdirection of the device while the source and drain are on the two endsof fingers. In another embodiment the gate fingers are oriented in thehorizontal and longer direction, as shown in FIG. 26. In such anembodiment, for large current rating LGA devices, in order to reservespace for the LV blocks (i.e., the regulator and amplifier/driver), thetwo parallel rectangular power transistors are separated by thehorizontal line A-B and LV blocks are above the larger main transistor.

Simulations were performed for the embodiment of FIG. 23 and layoutaccording to FIG. 24 based on a power HEMT with 8 A rating.

FIG. 27A shows the simulated input voltage pulse and FIG. 27B shows theauxiliary voltage supply output Vcc of about 5.5 V. External Vcci wasset at 10 V. In this embodiment, the channel width of the currentsupplying transistor (M60) was set at 6000 μm. It can be seen that anacceptable voltage regulation was achieved, and it is expected that thiscan be improved using a larger M60 channel width, at the cost of morewafer area.

FIGS. 28A and 28B show the simulated gate driving voltage and gatecurrent, respectively, for the main power EHEMT with nominal maximumcurrent rating of 8 A. These results, together with the results shown inFIGS. 22A and 22B, confirm that the regulator achieves the objective ofan auxiliary power supply capable of using a wide range of DC voltageinput. Thus, the embodiments provide a small die-area voltage regulatorsuitable for supplying approximately 6 V auxiliary power to a GaN IC.The input voltage range is sufficiently large to be compatible with allcurrent power system auxiliary power supplies. The embodiments make itpossible to implement GaN power devices into existing power systems.

In another embodiment a voltage regulator module such as that shown inFIG. 16 may be implemented as a gate driver in a source-followerconfiguration, wherein the source of the source follower amplifier isused to drive the gate of the main power transistor. Such an embodimentis shown in FIG. 29A, wherein the source of M60 drives the gate of thepower HEMT M70. The dashed arrow shows the direction of the reverserecovery current. The reverse recovery current in the source followeramplifier may be designed to effectively discharge the gate current ofthe main power transistor M70.

FIG. 29B shows an alternative embodiment implemented with an amplifier Ato drive the gate of the power transistor M70. In this embodiment abootstrap diode Db and capacitor Cb are used to maintain power to theamplifier for a short time during the discharge of the power transistorM70. This eliminates the need for an auxiliary power supply and servesas self-driven power switch. In the embodiments of FIGS. 29A and 29B,the source follower amplifier allows the circuits to achieve aself-driven condition.

FIG. 30 shows an embodiment of the circuit of FIG. 29B implemented witha DCFL amplifier (M22, M23, M24, M25) to drive the power transistor M70.A simulation of this circuit was conducted using APSYS™ software, withVdd=400 V and Rds=0.18 Ohm, and a 1 MHz PWM input signal of 15 V. FIGS.31A and 31B show switching transition waveforms (turn-off and turn-on,respectively) with excellent performance and a fast switching time ofabout 5 ns.

The embodiments of FIGS. 29A and 29B and FIG. 30 may be used, forexample, in place IGBT-based and MOSFET-based gate drivers inimplementations with suitable driving voltages (e.g., 12-15 V) andsufficient driving current. The gate driver embodiments areself-driving, thereby eliminating the need to for auxiliary powersupplies. This results in significant savings when upgrading fromsilicon power devices to GaN power devices. The embodiments may beimplemented completely or partially in GaN integrated circuit technologyusing, for example, the techniques described above.

FIG. 32 shows an application of a self-driven GaN power switch such asthe embodiments of FIG. 29A, 29B, or 30. In FIG. 32, the synchronousrectifiers SR1, SR2 of an active clamped converter are each implementedwith a such a self-driven GaN power switch. The load side of theconverter has a voltage that is usually higher than approximately 6 Vrequired by a GaN switch and the current in the load side is usuallysufficiently large so that use of such a self-driven GaN power switch issuitable. Advantageously, use of a self-driven GaN power switcheliminates the need for an auxiliary power supply, which reduces thecost and size of the converter. In the embodiment of FIG. 32, theself-driven GaN power switches used for the synchronous rectifiers SR1,SR2 are shown enclosed with dashed lines, indicating that they may beimplemented completely or partially as GaN integrated circuits. It willbe appreciated that self-driven GaN power switches as described hereinmay be used in other converters that involve synchronous rectification,such as, for example, flyback converters and DC-DC converters.

EQUIVALENTS

While the invention has been described with respect to illustrativeembodiments thereof, it will be understood that various changes may bemade to the embodiments without departing from the scope of theinvention. Accordingly, the described embodiments are to be consideredmerely exemplary and the invention is not to be limited thereby.

1. A gate driver circuit for a gallium nitride (GaN) power transistor,comprising: an input point that receives an input voltage and an outputpoint that outputs an output voltage to drive the GaN power transistor;a series circuit comprising at least one GaN D-mode HEMT (DHEMT) withgate-to-source connection and at least first to fourth GaN E-mode HEMTs(EHEMTs) each with drain-to-gate connection; an output EHEMT having adrain connected to the input point and a source connected to the outputpoint; wherein a drain of the DHEMT is connected to the input point anda source of the DHEMT is connected to a drain of the first EHEMT and toa gate of the output EHEMT; wherein a source of the first EHEMT isconnected to a drain of the second EHEMT, a source of the second EHEMTis connected to a drain of the third EHEMT, a source of the third EHEMTis connected to a drain of the fourth EHEMT, and a source of the fourthEHEMT is connected to a circuit common.
 2. The gate driver circuit ofclaim 1, wherein the output EHEMT comprises a source follower amplifier.3. The gate driver circuit of claim 1, further comprising an amplifier;wherein the output point is connected to an input of the amplifier;wherein an output of the amplifier drives the GaN power transistor. 4.The gate driver circuit of claim 3, wherein the amplifier inputcomprises a bootstrap configuration.
 5. The gate driver circuit of claim4, wherein the bootstrap configuration includes a diode connected inseries between the output point and the amplifier input, and a capacitorconnected between the amplifier input and the circuit common.
 6. Thegate driver circuit of claim 3, wherein the amplifier comprises a directcoupled FET logic (DCFL) amplifier.
 7. The gate driver circuit of claim1, comprising a GaN power transistor; wherein the GaN power transistoris self-driven.
 8. The gate driver circuit of claim 3, comprising a GaNpower transistor; wherein the GaN power transistor is self-driven. 9.The gate driver circuit of claim 1, implemented in GaN integratedcircuit technology.
 10. The gate driver circuit of claim 3, implementedin GaN integrated circuit technology.
 11. A power converter comprisingone or more GaN power switches; wherein at least one GaN power switchcomprises the gate driver circuit of claim
 1. 12. The power converter ofclaim 11, wherein the GaN power switch is a synchronous rectifier of thepower converter.
 13. A method for a gate driver for a gallium nitride(GaN) power transistor, comprising: providing an input point thatreceives an input voltage and an output point that outputs an outputvoltage to drive the GaN power transistor; connecting a series circuitcomprising at least one GaN D-mode HEMT (DHEMT) with gate-to-sourceconnection and at least first to fourth GaN E-mode HEMTs (EHEMTs) eachwith drain-to-gate connection; connecting an output EHEMT having a drainconnected to the input point and a source connected to the output point;wherein a drain of the DHEMT is connected to the input point and asource of the DHEMT is connected to a drain of the first EHEMT and to agate of the output EHEMT; wherein a source of the first EHEMT isconnected to a drain of the second EHEMT, a source of the second EHEMTis connected to a drain of the third EHEMT, a source of the third EHEMTis connected to a drain of the fourth EHEMT, and a source of the fourthEHEMT is connected to a circuit common.
 14. The method of claim 13,wherein the output EHEMT comprises a source follower amplifier.
 15. Themethod of claim 13, further comprising connecting the output point to aninput of an amplifier; wherein an output of the amplifier drives the GaNpower transistor.
 16. The method of claim 13, wherein the GaN powertransistor is self-driven.
 17. The method of claim 13, comprisingimplementing the gate driver using GaN integrated circuit technology.18. The method of claim 15, comprising implementing the gate driverusing GaN integrated circuit technology.
 19. The method of claim 13,comprising driving a GaN power transistor of a converter circuit. 20.The method of claim 19, wherein the GaN power transistor is asynchronous rectifier of the converter circuit.